1. Field of the Invention
This invention relates to a method for preparing powdered rubber. More particularly, it relates to a method for preparing powdered rubber with non-tackiness by treating a coagulated rubber with a carbon black-rich mixture of a rubber latex and an aqueous carbon black slurry in the presence of a coagulant.
2. Description of the Prior Art
There has been known and reduced into practice a method which comprises coagulating a mixture of a rubber latex, an oil emulsion and an aqueous carbon black slurry, dewatering and drying the coagulated rubber to obtain lumpy rubber, pressing the lumpy rubber in the form of bales, and packing the bales with polyethylene film. In this method, it is essentially required to form the lumpy rubber into bales and to pack each of the bales with polyethylene film since the coagulated rubber lumps are sticked with one another during dewatering and drying operations and also during storage or transportation to cause blocking. When the baled rubber is employed in a rubber processing work such as of tire manufacturing, it must be broken into pieces for automatic weighing purpose. Accordingly, if the rubber of the class mentioned above is supplied in the form of powder, not only the labor and time required for breaking the baled rubber into pieces in the rubber processing work can be saved and accordingly the automatic weighing operation is made easy, but also the powdered rubber can be dry blended with sulfur, an antioxidant, a cure accelerator, and other additives and directly fed to an extruder. This makes it possible to simplify the steps of producing rubber products to a considerable extent. Thus, the powdering of rubber has a significant meaning from an industrial point of view.
In the powdering of rubber, it is the common practice to treat rubber crumbs with an antiblocking agent so as to prevent the rubber crumbs from being sticked with one another. A number of methods for the preparation of non-tacky powdered rubber have been proposed, including a method wherein SBR is treated with powder of a resinous aromatic vinyl polymer to obtain non-cohesive granules of SBR, a method using talc for surface coverings, a method using metal salts or metal oxides for mixing with rubber to impart non-tackiness to the rubber, a method using polyethylene, polypropylene or the like to form dry coatings on rubber crumb surfaces, and the like methods. However, these powdering methods or techniques involve many problems that dust is undesirably produced during the powdering treatment and that a relatively large amount of powder material is required to impart an antiblocking property to rubber crumbs, giving an adverse influence on physical properties of the rubber.
Further, there is known a method wherein a crumb slurry containing an uncoagulated rubber latex is added and treated with an aqueous carbon black slurry to antiblock the crumbs U.S. Pat. No. 3,767,605). However, the method is encountered with difficulties that it is hard to exactly control the amount of the residual uncoagulated rubber latex and that the aqueous carbon black slurry must be limited in amount, so that it is difficult to impart the antiblocking property to the crumbs in a stable manner.
Under these circumstances, we have made an extensive study of a method for conveniently producing powdered rubber from a rubber latex with or without addition of carbon black and oil. As a result, it has been found that the blocking tendency of powdered rubber can be prevented by covering surfaces of crumbs with a carbon black-rich mixture of rubber and carbon black, while maintaining, as it is, the shape of crumbs formed upon coagulating a rubber latex.